The present invention relates to a glass composition which is capable of being pressed under a softened condition by heating and being chemically strengthened by alkali ion exchange. Such glass composition is suitable for obtaining glass articles such as optical glass of lenses and compact substrate glass. The present invention is more particularly relates to a substrate glass for information recording media that can be suitably used as a magnetic recording medium and so forth. In fact, this substrate glass is prepared by pressing and then chemical strengthening.
Substrate glass for information recording media, for example, is required to be able to be easily melted at a temperature comparable or lower than soda lime silica glass, have a softening temperature lower than soda lime silica glass, be able to be pressed easily and allow the formation of subtle surfaces that follow the press mold, be resistant to breakage as a result of having a thermal expansion coefficient similar to the press die, and have a high degree of so-called chemical strengthening by alkali ion exchange.
Furthermore, although it was common in the past to search for a method for grinding press formed glass to obtain surface smoothness, grinding is not preferable since it requires considerable time and trouble and has high costs. Although there have also been examples advocating the formation of a smooth surface by float process, although the float process is suitable for large-volume production of relatively large size glass such as window glass, it is not suited to manufacturing small, precise substrates such as substrates for information recording media.
Although the specification of U.S. Pat. No. 4,156,755 discloses a SiO2xe2x80x94Al2O3xe2x80x94Li2Oxe2x80x94Na2O glass additionally containing ZrO2 that is strengthened by ion exchange, this glass has an extremely high content of Al2O3 and ZrO2, has high melting and forming temperatures and has problems with melting ease and formability (particularly with respect to press forming).
Japanese Patent Laid-Open Publication No. 10-1329 discloses a glass composition for chemical strengthening and chemically strengthened glass article comprising SiO2xe2x80x94Al2O3xe2x80x94Li2Oxe2x80x94Na2Oxe2x80x94CaO, does not contain ZrO2 and is suitable for float process. In this publication, there is no mention of press forming, and both the temperature at which the glass exhibits a viscosity of 102 poises and the temperature at which the glass exhibits a viscosity of 104 poises are high. In addition, due to the absence of ZrO2, which is known to be a component for providing a glass with excellent water resistance and weather resistance, this glass is believed to be difficult to satisfy water resistance and weather resistance.
Japanese Patent Laid-Open Publication No. 5-32431 discloses a glass, which is to be subjected to chemical strengthening, comprising SiO2, Al2O3, Li2O, Na2O and ZrO2, and a chemically strengthened glass produced thereby. In this publication, there is no descriptions of the temperature at which the glass exhibits a viscosity of 102 poises, the temperature at which the glass exhibits a viscosity of 104 poises, the softening point and the like of this glass which is to be subjected to chemical strengthening. In addition, although the ZrO2 component is contained at a relatively high range, this tends to make the glass melting temperature higher and forming, and particularly press forming, difficult.
Japanese Patent Laid-Open Publication No. 10-158028 discloses a glass substrate for magnetic discs, comprising SiO2, Al2O3, Na2O, K2O, RO and ZrO2, where RO is MgO, CaO, SrO or BaO, that is resistant to scratching and suitable for float process. Press forming of this glass is not mentioned in this publication, and the temperature at which the glass exhibits a viscosity of 102 poises and the temperature at which the glass exhibits a viscosity of 104 poises are both high. In addition, although the K2O content is relatively high in comparison with the Na2O content, in consideration of the fact that the sodium component in the glass is ion-exchanged by molten potassium salt, efficient ion exchange is thought to be difficult.
Japanese Patent Laid-Open Publication No. 11-199267 discloses a glass substrate for magnetic discs, consisting essentially of SiO2, Al2O3, Li2O, Na2O, K2O, MgO, CaO, ZrO2 and TiO2.
Namely, there have been cases in the prior art having problems with formability including melting ease and press forming, inadequate weather resistance, moisture resistance and water resistance, or being unable to be chemically strengthened by ion exchange.
It is therefore an object of the present invention to provide a lithium-sodium-aluminosilicate glass composition that is capable of providing an ion exchange strengthened glass article through it pressing and then alkali ion exchange.
It is another object of the present invention to provide such ion exchange strengthened glass article.
It is a further object of the present invention to provide a process for producing such ion exchange strengthened glass article.
According to the present invention, there is provided a lithium-sodium-aluminosilicate glass composition comprising a softening point of 700xc2x0 C. or lower; a first temperature at which the glass composition exhibits a viscosity of 102 poises, the first temperature being 1450xc2x0 C. or lower; a second temperature at which the glass exhibits a viscosity of 104 poises, the second temperature being 1000xc2x0 C. or lower; and a mean thermal expansion coefficient between 30xc2x0 C. and a glass transition temperature of the glass composition, the coefficient being within a range of from 90xc3x9710xe2x88x927/xc2x0 C. to 130xc3x9710xe2x88x927/xc2x0 C.
According to the present invention, there is provided an ion exchange strengthened glass article prepared from the glass composition by a process. This process includes (a) providing the glass composition; (b) pressing the glass composition at the softening point, thereby to obtain a precursor of the glass article; and (c) bringing the precursor into contact with a molten salt comprising a potassium salt, thereby conducting an alkali ion exchange strengthening.
The glass composition of the invention can be melted easily, can be easily formed by pressing, allows the formation of subtle surfaces that closely follow the press mold, is resistant to breakage by closely approximating the thermal expansion coefficient of the press mold, and can be chemically strengthened at high degree by alkali ion exchange, and more particularly, provides a substrate glass for information recording media that can be preferably used as a magnetic recording medium and so forth.
A pressed glass composition (i.e., a precursor of an ion exchange strengthened glass article) of the present invention is produced in two stages by first melting, clarifying and homogenizing the glass raw materials, producing a preliminary formed glass having a shape that approximates the target shape by, for example, casting the resulting molten glass, and then re-heating this preliminary formed glass and supplying this to press forming. Alternatively, the pressed glass composition of the present invention can be produced in a continuous operation by first producing the above-mentioned preliminary formed glass and while still heated, supplying it to press forming after slightly adjusting the temperature.
When glass exhibits a viscosity of 102 poises during its melting, the glass becomes extremely rich in fluidity This is advantageous for melting and homogenizing of the glass. Thus, the temperature at which glass exhibits a viscosity of 102 poises is also referred to as the melting temperature. Common soda lime silica glasses are considered to melt easily, and the temperature at which these glasses exhibit a viscosity of 102 poises is within the range of 1400-1450xc2x0 C. Similarly, the temperature at which the glass composition according to the present invention exhibits a viscosity Of 102 poises is 1450xc2x0 C. or lower.
With respect to the temperature for the above-mentioned preliminary forming, namely the working temperature, a glass viscosity of 104 poises is the viscosity that serves as an index in terms of heat forming the glass composition into the shape of a plate or container and so forth, and this also applies to the preliminary forming in the present invention. The above-mentioned temperature at which common soda lime silica glasses exhibit a viscosity of 104 poises is roughly 1000xc2x0 C. Similarly, the temperature at which the glass composition according to the present invention exhibits a viscosity of 104 poises is 1000xc2x0 or lower.
In the invention, the base material of a press die for the press forming is preferably a material having excellent heat resistance and abrasion resistance, Examples of such material include alloy steel (e.g., austenite steel), cermet (e.g., TiCxe2x80x94Moxe2x80x94Ni-based cermet), ceramics (e.g., alumina and zirconia) and heat-resistant glass (e.g., aluminosilicate glass such as calcium-aluminosilicate glass). The base material of a press die is formed to the desired shape and optical surface and polished. Then, the surface of the press die can be covered with a thermally and chemically stable thin film which allows the heat-softened glass composition to adhere to the film. The thin film is preferably resistant to corrosion caused by the heat-softened glass composition. The film may be a precious metal film, a tungsten film, a tantalum film or an alloy film of these metals and can be formed by, for example, sputtering.
Conventionally, the surface of the pressed glass is polished after obtaining its optical surface. In contrast, the desired optical surface can be obtained by performing the above-mentioned procedure, without a polishing step.
Naturally, it is preferable that the glass composition has thermal properties and other characteristics suitable for press forming for obtaining the above-mentioned optical surface, and moreover, it is also preferable to have characteristics suitable for performing ion exchange chemical strengthening.
The viscosity of the glass composition in press forming is preferably adjusted to falling within a range of 107.6-1012 poises, and more preferably in the vicinity of 1010 poises. In fact, 107.6 poises corresponds to softening point of the glass composition. In other words, the glass composition exhibits a viscosity of 107.6 poises at its softening point. In the case of viscosity lower than 107.6 poises, the glass may easily adhere to the surface of the press die and may not be easily separated. This may result in greater susceptibility to breakage of the glass during its separation from the press die. In addition, the press may wear rapidly. At a viscosity higher than 1012 poises, it may become difficult for the glass composition to adhere to the surface of the press die. With this, it may become difficult to obtain an optical surface that precisely follows the surface of the press die.
With respect to the temperature during the above-mentioned press forming, although the above-mentioned press base material and thin film may have excellent heat resistance, abrasion resistance and corrosion resistance, when pressing is performed repeatedly at high temperatures, the optical surface may be susceptible to damage by abrasion. The temperature, at which a common soda lime silica glass exhibits a viscosity of 107.6 poises (softening point), is on the order of 720-740xc2x0 C. In contrast, it is preferably 700xc2x0 C. or lower for the glass composition of the present invention. Furthermore, it is preferably 600xc2x0 C. or lower in the invention at a more preferable viscosity, namely 1010 poises.
The pressing pressure is preferably within the range of 10 Kgf/cm2 to 50 Kgf/cm2 (approximately 1-5 MPa). If the pressing pressure is increased to a high pressure in excess of 50 Kgf/cm2, the glass composition may become susceptible to cracking and breakage, or the press itself may break. If the pressing pressure is decreased to a low pressure less than 10 Kgf/cm2, it may become difficult to obtain an optical surface that matches the surface of the press die.
Within the above-mentioned ranges of glass viscosity (temperature) and pressing pressure, the formed glass adheres well to the press die (thin film). Thus, an optical surface can be obtained that precisely matches the optical surface of the above-mentioned base material of the press die.
It is preferable that the thermal expansion coefficient of the glass composition approximates that of the press die. Thus, the thermal expansion coefficient of the former is suitably set between 90xc3x9710xe2x88x927/xc2x0 C. and 120xc3x9710xe2x88x927/xc2x0 C. as the mean value between room temperature and the glass transition temperature. For example, in the case that the press die is made of a material selected from heat-resistant steel and a majority of heat-resistant ceramics, the thermal expansion coefficient can be roughly from 100xc3x9710xe2x88x927/xc2x0 C. to 120xc3x9710xe2x88x927/xc2x0 C., and in the case of calcium-aluminosilicate glass, roughly from 90xc3x9710xe2x88x927/xc2x0 C. to 100xc3x9710xe2x88x927/xc2x0 C.
Chemical strengthening by alkali ion exchange for the pressed glass composition can be performed by immersing the glass in a molten salt containing a potassium salt at a temperature that is equal to or lower than the glass transition temperature, at which thermal deformation of the glass is difficult, and equal to or higher than the melting point of sodium nitrate and/or potassium nitrate, and performing ion exchange for several hours. A substrate glass for information recording media is generally required to have a three-point bending strength following chemical strengthening of 1500 Kgf/cm2 or more (approximately 147 MPa or more). In contrast, in the present invention, a value of 2000 Kgf/cm2 or more (approximately 196 MPa or more) is preferable in consideration of further improving strength and ease of handling of the substrate glass.
As stated above, the glass composition according to the present invention is a lithium-sodium-aluminosilicate glass, and the silica component is essential for forming glass. Although the alumina component is also for forming the glass together with the silica component, it is necessary for increasing the ion exchange rate during chemical strengthening and for improving water resistance of the glass following chemical strengthening. The lithium and sodium components are necessary for facilitating melting of the glass raw materials and are effective in lowering the viscosity, softening point and glass transition temperature of the glass composition at high temperatures. The lithium component (lithium ions) in particular is essential for forming a surface compressed layer in the glass article by ion exchange with larger ion diameter sodium and potassium ions, while the sodium component (sodium ions) is essential for strengthening by similarly forming a surface compressed layer in the glass article by ion exchange with larger ion diameter potassium ions.
The glass composition preferably contains BaO and ZrO2. BaO facilitates melting of the glass while also lowering the viscosity, softening point and glass transition temperature of the glass at high temperatures. The glass thermal expansion coefficient may increase resulting in poor water resistance and weather resistance when the above-mentioned lithium, sodium and other alkaline components are present in the lass in larger amounts than what is necessary. In contrast, the thermal expansion coefficient can be properly adjusted by decreasing the amount of alkaline components and by introduction of BaO instead, thereby improving water resistance and weather resistance. In addition, ZrO2 is effective in increasing the ion exchange rate during chemical strengthening, and significantly improves water resistance and weather resistance of the glass following chemical strengthening.
The following components can be contained in the glass composition according to the invention. SiO2 is the main component that forms the glass, and in terms of percent by weight, may be contained within the range of 58-66% in the glass. If the content of SiO2 is less than 58%, glass forming may be no longer easy, devitrification may occur easily and the water resistance of the glass following ion exchange chemical strengthening may be inferior. If the SiO2 content exceeds 66%, the glass melting temperature increases, and the forming temperature increases for forming the glass into a shape, including preliminary forming, and particularly for press forming. Although Al2O3 is a component that forms the glass together with SiO2, it is essential for increasing the ion exchange rate during chemical strengthening and for improving the water resistance of the glass following chemical strengthening. Al2O3 may be contained in the glass composition within the range of 7-15%. If the content of Al2O3 is less than 7%, the above-mentioned advantageous effects may become insufficient. If the content of Al2O3 exceeds 15%, the glass viscosity increases and thereby the temperatures of preliminary forming and press forming must be increased. In addition to LiO2 being essential for performing chemical strengthening by ion exchange, it is essential for facilitating glass melting and forming at relatively low temperatures, and can be contained in the glass within the range of 4.7%. If the content of LiO2 is less than 4%, the above-mentioned melting and forming temperatures rise. If the content of LiO2 exceeds 7%, the glass viscosity is lowered more than what is required, and the water resistance of the glass after chemical strengthening by ion exchange may become poor. Similar to LiO2, Na2O is essential for performing chemical strengthening by ion exchange, and facilitates glass melting and forming. In addition, it can be obtained at a lower cost than other alkali metal component raw materials. Na2O may be contained in the glass within the range of 10-15%. If the content of Na2O is less than 10%, glass viscosity increases and thereby the above-mentioned melting and forming temperatures must be increased. If the content of Na2O exceeds 15%, glass viscosity lowers beyond that what is necessary and the water resistance of the glass after chemical strengthening by ion exchange may become poor. K2O is suitably introduced as necessary for increasing glass meltability and for adjusting the glass viscosity and thermal expansion coefficient. Furthermore, if the content of K2O in the glass exceeds 3%, the thermal expansion coefficient may become excessively large which causes the glass viscosity to be lowered beyond that which is necessary. Consequently, the content of K2O in the glass is preferably 3% or less. MgO is suitably introduced as necessary for enhancing glass meltability and adjusting the viscosity and thermal expansion coefficient of the glass. Similar to CaO, MgO offers the advantage of being able to be obtained at a lower cost than alkali metal component raw materials. However, since the ion exchange rate is lowered if MgO is contained in excess of 4% in the glass, the content of MgO is preferably 4% or less. CaO is also a component that is suitably introduced as necessary for enhancing glass meltability and adjusting the viscosity and thermal expansion coefficient of the glass. Furthermore, since the ion exchange rate is lowered if CaO is contained in excess of 4% in the glass, the content of CaO is preferably 4% or less. BaO is a component that is effective for enhancing glass meltability and lowering the liquid phase temperature of the glass. In addition, BaO lowers the forming temperature and facilitates forming, without making the thermal expansion coefficient of the glass excessively high, as compared with alkali metal components such as Na2O and K2O. Furthermore, BaO does not worsen water resistance of the glass, as compared with the above-mentioned alkali metal components. BaO may be contained in the glass within the range of 1-10%. If the content of BaO is less than 1%, the above-mentioned actions and advantageous effects may become insufficient, There is no need to contain BaO in excess of 10%, and on the contrary, this content of BaO tends to increase the thermal expansion coefficient of the glass. TiO2 is a component that is suitably contained for adjusting the viscosity and thermal expansion coefficient of the glass, or improving the chemical durability of the glass following chemical strengthening by ion exchange. However, glass viscosity may increase too much resulting in poor glass melting and formability if the content of TiO2 in the glass exceeds 5%. Therefore, the content of TiO2 is preferably 5% or less. ZrO2 serves for increasing the ion exchange rate and improving water resistance after ion exchange, and is preferably contained in the glass within the range of 1-6%. If the content of ZrO2 is less than 1%, its actions and advantageous effects may become insufficient. The content of ZrO2 exceeding 6% is not preferable, since glass viscosity increases and melting and forming temperature rises.
A first glass composition according to a first preferred embodiment of the present invention contains 58-65% SiO2, 10-15% Al2O3, 4-6% Li2O, 10-13% Na2O, 0-3% K2O, 0-4% Mg,O, 0-4% CaO, 0-2% SrO, 1-10% BaO, 0-5% TiO2 and 1-5% ZrO2. Furthermore, the first glass composition has a mean thermal expansion coefficient between 30xc2x0 C. and the glass transition temperature of from 90xc3x9710xe2x88x927/xc2x0 C. to 120xc3x9710xe2x88x927/xc2x0 C. A second glass composition according to a second preferred embodiment of the present invention contains 59-66% SiO2, 7-12% Al2O3, 4-7% Li2O, 12-15% Na2O, 0-3% K2O, 0-4% MgO, 0-4% CaO, 0-2% SrO, 1-10% BaO, 0-5%/ TiO2 and 3-6% ZrO2. Furthermore, according to the second glass composition, the mean thermal expansion coefficient between 30xc2x0 C. and the glass transition temperature is from 100xc3x9710xe2x88x927/xc2x0 C. to 130xc3x9710xe2x88x927/xc2x0 C., and the glass transition temperature is 481xc2x0 C. or lower.